The electrophoretic effect is well known and the prior art is replete with a number of patents and articles which describe the effect. As will be recognized by a person skilled in the art, the electrophoretic effect operates on the principle that certain particles, when suspended in a medium, can be electrically charged and thereby caused to migrate through the medium to an electrode of opposite charge. Electrophoretic image displays (EPIDs) utilize the electrophoretic effect to produce desired images.
EPIDs generally comprise a suspension of colored charged pigment particles dispersed in a dyed solvent of contrasting color, which is injected into a cell consisting of two parallel and transparent conducting electrode panels. The charged particles are transported and packed against one electrode under the influence of an electric field, so that the viewer may see the color of the pigment. When the polarity of the field is reversed, the pigment particles are transported and packed on the opposite electrode. If the optical density of the dyed solvent is high enough to absorb the light scattered by the particles residing on the rear electrode, the observer will perceive the color of the dyed solvent. The performance of the resulting display is strongly dependent upon the suspension stability.
In non-aqueous dispersions colloid particles generally owe their stability to the fact that their surfaces are charged and, hence, repel each other. When the particles are uncharged, the dispersion is unstable. The fact that a colloidal particle bears a net surface charge is not a sufficient condition for stability because electroneutrality demands that the particle plus its immediate surroundings bear no net charge. In other words, the surface charge must be compensated by an equal but opposite counter charge, so that surface charge and countercharge together form an electrical double layer. P. Murau and B Singer, in an article appearing in Vol. 49, No. 9 of the Journal of Applied Physics (1978) and entitled "The Understanding and Elimination of Some Suspension Instabilities in an Electrophoretic Display", indicate that when the double layer is compressed, the particles can approach each other to within a few hundred angstroms before repulsion is felt whereupon the van der Waals attraction becomes so strong that aggregation occurs.
The interactions of particle surfaces and charge control agents in colloidal suspensions has been the subject of considerable research. Reference is made to an article entitled "Mechanism of Electric Charging of Particles in Nonaqueous Liquids" appearing in Vol. 15 of the Journal of the American Chemical Society (1982), wherein F. M. Fowkes et al discuss the mechanism of electrostatic charging of suspended acidic particles by basic dispersants in solvents of low dielectric constant. Reference is also made to an article entitled "Steric and Electrostatic Contributions to the Colloidal Properties of Nonaqueous Dispersions" appearing in Vol. 21 of the Journal of the American Chemical Society (1984) wherein F. M. Fowkes and R. J. Pugh discuss the importance of anchoring sites for steric stabilizers in minimizing particle flocculation. The essential point developed by these references is that particle surface interactions are acid-base in character. Acidic pigment surface sites and basic charge control agents yield negative pigment surface charge. On the other hand, basic pigment surface sites and acidic charge control agents yield positive pigment surface charge.
Since electrophoretic devices utilize low polarity liquids in which ionization of ordinary organic acids and salts is negligible (approximately 10.sup.-10 moles per liter), the charge of the particle is governed by trace impurities unless otherwise controlled by adsorbing on the pigment surface a suitable charge control agent. This amount of charge, although sufficient for electrophoretic activity may still be inadequate for electrostatic stabilization of the suspension. If the charge control agent is also polymeric, or a polymeric dispersant is present in addition, the colloid stability can be further enhanced.
Over recent years, attention has therefore been directed to dispersion stabilization by way of adsorbed polymers on particle surfaces. If two colloidal particles coated with adsorbed layers of polymers approach each other, steric repulsion can occur as soon as the polymer layers start to penetrate. According to Murau and Singer, the polymer molecules adsorbed on a colloidal particle never lie flat on the surface. Rather, parts of the long chains (loose-ends, side branches, and loops) are free from the surface and surrounded by liquid. The overlapping of the polymer chains upon close approach can be pictured as a localized increase in the polymer concentration. This case is thermodynamically less favorable then the "dilute" situation existing when particles are far apart.
As will be recognized by a person skilled in the art, the selection of the electrophoretic particles used in the EPID is very important in determining the performance of the EPID and the quality of the viewed image produced. Ideally, electrophoretic particles should have an optimum charge/mass ratio, which is dependent upon the particle size and surface charge, in order to obtain good electrostatic deposition at high velocity as well as rapid reversal of particle motion when voltages change. Additionally, it is desirable to utilize electrophoretic particles that have essentially the same density as the fluid medium in which they are suspended. By using electrophoretic particles of essentially the same density as the suspension medium, the migration of the electrophoretic particles through the medium remains independent of both the orientation of the EPID and the forces of gravity.
To effect the greatest optical contrast between electrophoretic particles and the suspension medium, it is desirable to have either light-colored particles suspended in a dark medium or black particles suspended in a backlighted clear medium. In the prior art, it has been proven difficult to produce black electrophoretic particles that are dielectric, of uniform size and have a density matching that of a common suspension medium. As a result, EPIDs, commonly use readily manufactured light colored electrophoretic particles suspended in dark media. Such EPIDs are exemplified in U.S. Pat. Nos.: 4,655,897 to DiSanto et al., 4,093,534 to Carter et al., 4,298,448 to Muller et al., and 4,285,801 to Chaing. In such art, the light colored particles are commonly inorganic pigments. Titanium dioxide, for example, has been used in EPIDs to produce a good optical contrast between the white particles and the colored suspension medium. However, it has a density about 4 g/cm.sup.3 which is too high to match with any organic liquid to prevent the sedimentation problem. In the past decade, great effort has been made to solve the density problem of titanium dioxide. However, very little work has succeeded without trading off the quality of the images, especially in regard to the whiteness. Coating titanium dioxide particles with a polymeric material to reduce the density of titanium dioxide is an example.
It is an object of the present invention to produce stable suspensions suitable for use in EPIDs, the suspension also having high electrophoretic sensitivity. It is a further object to produce light colored dielectric particles which may be used in such suspensions.